1. Field of the Invention
The present invention relates to a wrench with a fixed maximum operational torque to prevent damage to an object secured by a fastener driven by the wrench.
2. Description of the Related Art
FIG. 13 of the drawings illustrates a conventional wrench 1 having a hexagonal driving portion with six planar faces 2 for engaging with six faces of a hexagonal groove in a top face of a fastener. However, slide tends to occur between the planar faces of the driving portion of the wrench 1 and the faces of the fastener. FIG. 14 illustrates a so-called TORX wrench 3 having plural arcuate faces 4 for engaging with respective arcuate faces in a top face of a fastener. Such a TORX wrench 3 is used to tighten important parts of a car and cutting tools. As illustrated in FIG. 15, a blade 7 is tightened to a cutting tool 5 by a bolt 6. However, it was found that the expensive blade 7 tends to be damaged when the bolt 6 is excessively tightened. But the blade 7 could fly away and thus cause injury if the bolt 6 is not tightened to the desired extent.
Taiwan Utility Model Publication No. 266533 discloses a wrench including a tubular member, a sounding means mounted in an intermediate portion of the tubular member, and a torque-setting means attached to a rear end of the tubular member. The torque-setting means includes a tube mounted around the rear end of the tubular member, and marks are provided on the tubular member for indicating the maximum torque value applied by the wrench. The maximum torque value of the wrench is set by means of turning the tube of the torque-setting means, which causes a change in the compressed extent of the elastic element and thus changes the force pressing against the sounding means, and the value is indicated by the marks on the tubular member. When the torque applied by the user to a fastener to be tightened is greater than the predetermined maximum torque value, the sounding means sounds, indicating that the fastener has already been tightened. However, the overall volume of the wrench is too large to be used in a small space. Further, the wrench cannot be used to drive fasteners of various types.